This invention relates to aluminum based materials, and, more particularly, to superplastically formable aluminum alloys and composites made from aluminum powders.
A continuing consideration in the development of materials for aircraft and spacecraft is the need for achieving higher stiffness and strength in materials of reduced weight, which are also microstructurally uniform, formable, joinable, producible, corrosion resistant, etc. Alloys and composites of aluminum have been developed to meet the many requirements for use in aerospace structures, and most aircraft now use these materials for ambient and moderate temperature structural applications. Because added weight in a flight structure results in severe penalties in performance and fuel costs over the life of the aircraft, reductions of only a few pounds in an aircraft, through use of improved materials, can have significant benefits that justify the added costs of the improved materials.
Aluminum alloys made by powder metallurgical techniques meet many of the requirements for aircraft structural use. Aluminum alloys are first processed into fine powders by melt atomization. The powders are consolidated into a solid structural form by pressure applied at elevated temperature. The processing through the powder form results in a refined microstructure having improved mechanical properties, and also provides a high degree of uniformity throughout a part. Consolidation to nearly the final required shape is often possible using powder techniques, so that machining costs and material waste are minimized.
The use of powder metallurgical techniques also permits the preparation of fine particulate composite materials. Composite materials are physical mixtures of two or more components which retain their physical distinctness after fabrication, unlike an alloy wherein the alloying elements are no longer distinct after the alloy is prepared. Composite materials allow the high stiffness and strength of certain finely divided reinforcements to be economically utilized by incorporating these reinforcements into a matrix which surrounds and protects the reinforcements, and contributes its own desirable properties. The composite material exhibits mechanical properties that are a mixture of those of the components, and careful selection of the matrix alloy and reinforcement results in improved composite properties with reduced structural weight.
Many aluminum alloys are commercially available in a powder form. Composite materials having an aluminum matrix and an incorporated reinforcement, prepared by powder metallurgical or casting techniques, are also available. More specifically, aluminum matrix composites with fine silicon carbide reinforcements, prepared by powder consolidation techniques, are in a development stage and can be obtained commercially.
While parts of consolidated aluminum powders, and consolidated mixtures of aluminum powders and reinforcement, have many advantages, their ductility is generally low, with uniform elongations for conventional powder alloys of 15 percent or less, and uniform elongations for a composite with 0.10 volume fraction of reinforcement of 6 percent or less. The low ductility results in poor formability in conventional forming operations which prepare shaped parts from the materials. These materials as currently fabricated also cannot be formed by superplastic forming, a manufacturing technique by which metals can be formed by processes somewhat similar to those used for plastics. To be suitable for superplastic forming, a metal must have a uniform elongation at the forming temperature of 300 to 500 percent or greater. If appropriate microstructures in aluminum powder alloys and composites can be developed, superplastically formable sheet stock can be economically prepared at a central mill for later use by aircraft manufacturers in forming aircraft skin structures and the like at their plants.
Accordingly, there exists a need for aluminum alloys and reinforced composites that exhibit high uniform elongations, and particularly superplastic properties. Such materials must have excellent stiffness and strength so that they impart high stiffness-to-weight and strength-to-weight ratios to the aircraft structure, and would desirably be superplastically formable to permit economical fabrication of parts. The materials must also be compatible with existing manufacturing and processing machinery and procedures. The present invention fulfills this need, and further provides related advantages.